Method for treating dry eye syndrome

ABSTRACT

A method of preventing or treating an eye condition includes administering a sterile aqueous ophthalmic solution to an eye for the prevention or treatment of dry eye syndrome or dry eye sign and/or dry eye symptom. The sterile aqueous ophthalmic solution includes about 0.05% to about 0.5% (w/w) of N—(N-acetylcysteinyl-)chitosan or a pharmaceutically acceptable salt thereof in a carrier solution, wherein the N—(N-acetylcysteinyl-)chitosan has a content of free thiol groups in an amount of from 80 μmol/g polymer to 280 μmol/g polymer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.15/308,797, filed Nov. 3, 2016, which is a 371 Application ofInternational Patent Application No. PCT/EP2015/059674, filed May 4,2015, which claims the benefit of European Patent Application No.14167381.4, filed May 7, 2014, the disclosures of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an aqueous ophthalmic solution. Thepresent invention also relates to a container containing the aqueousophthalmic solution. The present invention further relates to a methodof treating dry eye syndrome using the novel aqueous ophthalmicsolution. The method is useful in relieving dry eye signs and/orsymptoms. The method involves administering to a subject in need thereofan aqueous ophthalmic solution containing a thiolated chitosan.

BACKGROUND OF THE INVENTION

Dry eye syndrome (DES), also referred to as dry eye disease, is a highlyprevalent ocular surface disease. Approximately 40 million Americans areaffected with some type of dry eye, a significant portion of which thatare age 50 years and older have moderate-to-severe dry eye (Schaumberg,Sullivan et al., 2003, Prevalence of dry eye syndrome among US women, AmJ Ophthalmol (136): 318-326; Schaumberg, Dana et al., 2009, Prevalenceof dry eye disease among US men: estimates from the Physicians' HealthStudies, Arch Ophthalmol (127): 763-768).

Broadly, dry eye disease can be any syndrome associated with tear filminstability and dysfunction (such as increased tear evaporation and/orreduced aqueous secretion). Among the indications that are referred toby the general term “dry eye disease” are: Keratoconjunctivitis sicca(KCS), age-related dry eye, Stevens-Johnson syndrome, Sjogren'ssyndrome, ocular cicatrical pemphigoid, corneal injury, ocular surfaceinfection, Riley-Day syndrome, congenital alacrima, nutritionaldisorders or deficiencies (including vitamin deficiencies),pharmacologic side effects, glandular and tissue destruction, autoimmuneand other immunodeficient disorders, and inability to blink in comatosepatients. Also included are dry eye symptoms caused by environmentalexposure to airborne particulates, smoke, smog, and excessively dry air;as well as contact lens intolerance and eye stress caused by computerwork or computer gaming.

There are other diseases that have a high degree of co-morbidity withdry eye disease: Allergic conjunctivitis (seasonal and chronic),blepharitis and Meibomian gland dysfunction. These conditions affect thequality and stability of the tear film, which results in dry eye signsand symptoms.

Laser assisted vision correction procedures such as photorefractivekeratectomy (PRK), laser-assisted sub-epithelial keratectomy (LASEK) andlaser-assisted in situ keratomileusis (LASIK) also negatively influencetear film functionality and frequently cause (temporary) dry eyedisease.

Currently the management of DES encompasses both pharmacologic andnon-pharmacologic treatments, including environmental management,avoidance of exacerbating factors, lid hygiene, tear supplementation(artificial tears), secretagogues (to increase the production of tears),punctual plugs, anti-inflammatory agents (cyclosporine, steroids),moisture chamber, and even salivary gland auto transplantation (Behrens,Doyle et al., 2006, Dysfunctional tear syndrome: a Delphi approach totreatment recommendations, Cornea (25): 900-907). Currently availableoptions for treating DES are inadequate. Even tear supplementation isnot an ideal treatment option as it requires the subject to repeatartificial tear installation very many times during the day.

Various polymers have been disclosed as possible aids in providing somebenefit to alleviating DES symptoms and in fact some artificial tearscontain one or more polymers, including the currently top 5 best sellingover-the-counter (OTC) products for dry eye within the EU (Celluvisc®,Systane®, Hylo-Comod®, Optive® and Artelac®). These polymers areintended to protect ocular mucous membranes and provide lubrication forthe ocular surface. Examples include cellulose derivatives, hyaluronicacid, liquid polyols, polyvinyl alcohol, povidone, carbopol andhydroxypropyl-guar. Polymers used in products to treat DES haverelatively short residence time on the ocular surface and requirefrequent instillation. In order to increase ocular residence time, someformulations contain petroleum jelly or mineral oil; however, due tosignificant blurring these highly viscous products can only be used inthe evening prior to sleep. (Abelson et al., 2008, Tear Substitutes. In:Albert and Miller, eds. Principles and Practices of Ophthalmology, 3rdedition, vol. 1. Philadelphia: W.B. Saunders Company, 287-292). Allother tear substitutes have to be instilled repeatedly during the day.

Some potential improvements to these polymers have been disclosed. Onepotential improvement could be to use a polymer that has significantmucoadhesive properties in order to increase residence time of theformulation on the ocular surface without causing significant blurring.Chitosan, a polycationic polymer which is derived from the naturalpolymer chitin, is well known for its mucoadhesive properties. Ocularresidence time of ophthalmic formulations containing chitosan can beincreased not only due to its viscosity enhancing properties but alsobecause of interactions of chitosan with negatively charged mucins onthe ocular surface (Wadhwa, Paliwal et al., 2009, Chitosan and its rolein ocular therapeutics, Mini Rev Med Chem (9): 1639-1647). In addition,chitosan has antimicrobial activity against various pathogenicmicroorganisms (Felt, Carrel et al., 2000, Chitosan as tear substitute:a wetting agent endowed with antimicrobial efficacy, J Ocul PharmacolTher (16): 261-270; Dai, Tanaka et al., 2011, Chitosan preparations forwounds and burns: antimicrobial and wound-healing effects, Expert RevAnti Infect Ther (9): 857-879).

Thiolation of polymers has been disclosed to further increase theirmucoadhesive properties. EP 1126881 B1 discloses a mucoadhesive polymercomprising at least one non-terminal thiol group. The use of thiolatedpolysaccharides for preparing an implant for tissue augmentation isdisclosed in WO 2008/077172, wherein said thiolated polymers arecharacterised by the formation of disulfide bonds which leads to astabilisation of the polymeric network. The priority application of WO2008/077172, A 2136/2006, discloses further application fields forthiolated polymers.

Modification of chitosan by covalent attachment of thiol group bearingligands (i.e., thiolation) has been disclosed. It has also beendisclosed that thiolation increases the mucoadhesive properties ofchitosan (Kast and Bernkop-Schnurch, 2001, Thiolated polymers—thiomers:development and in vitro evaluation of chitosan-thioglycolic acidconjugates, Biomaterials (22): 2345-2352; Bernkop-Schnurch, Hornof etal., 2004, Thiolated chitosans, Eur J Pharm Biopharm (57): 9-17;Bernkop-Schnurch, 2005, Thiomers: a new generation of mucoadhesivepolymers, Adv Drug Deliv Rev (57): 1569-1582; Schmitz, Grabovac et al.,2008, Synthesis and characterization of a chitosan-N-acetyl cysteineconjugate, Int J Pharm (347): 79-85). The antimicrobial efficacy of somethiolated chitosans was evaluated as well (WO2009132226 A1; WO2009132227A1; WO2009132228 A1; Geisberger, Gyenge et al., 2013,Chitosan-thioglycolic acid as a versatile antimicrobial agent,Biomacromolecules (14): 1010-1017)

N-acetylcysteine (NAC) is a derivative of the thiol group bearing aminoacid L-cysteine. NAC is a reducing agent with antioxidative activity. Itis also well known for its ability to reduce mucus viscosity by reducingmucin disulfide bonds. Due to these mucolytic properties NAC is widelyused to reduce mucus viscosity in broncho-pulmonary disorders withexcessive mucus production. Topical ophthalmic formulations containingthe mucolytic and antioxidant agent NAC are used for the treatment ofcorneal diseases such as meibomian gland dysfunction and DES (Lemp,2008, Management of dry eye disease, Am J Manag Care (14): S88-101;Akyol-Salman, Azizi et al., 2010, Efficacy of topical N-acetylcysteinein the treatment of meibomian gland dysfunction, J Ocul Pharmacol Ther(26): 329-333). EP 0 551 848 B1 discloses an ophthalmic pharmaceuticalcomposition for the treatment of DES containing NAC in a concentrationbetween 3% and 5% (w/v) and polyvinylalcohol.

It has been disclosed that thiolation of chitosan using NAC increasesits ocular residence time on rabbit eyes when compared withnon-thiolated chitosan (Dangl, Hornof et al., 2009, In vivo Evaluationof Ocular Residence Time of ¹²⁴I-labelled Thiolated Chitosan in RabbitsUsing MicroPET Technology, ARVO Meeting Abstracts (50): 3689).

It has been disclosed that N—(N-acetylcysteinyl-)chitosan HCl has somebeneficial effect on the ocular surface of the mouse eye in mouse dryeye models (Hongyok, Chae et al., 2009, Effect ofchitosan-N-acetylcysteine conjugate in a mouse model of botulinum toxinB-induced dry eye, Arch Ophthalmol (127): 525-532; Hornof, Goyal et al.,2009, Thiolated Chitosan for the Treatment of Dry Eye—Evaluation in MiceUsing the Controlled-Environment Chamber Model, ARVO Meeting Abstracts(50): 3663).

Further publications reviewing and discussing various uses of thiolatedpolymers are listed below:

-   Hornof et al., Mucoadhesive ocular insert based on thiolated    poly(acrylic acid): development and in vivo evaluation in humans;    Journal of Controlled Release 89 (2003) 419-428;-   Hornof, M., In vitro and in vivo evaluation of novel polymeric    excipients in the ophthalmic field, Thesis, University of Vienna,    2003;-   Bernkop-Schnurch et al., Permeation enhancing polymers in oral    delivery of hydrophilic macromolecules: Thiomer/GSH systems, J.    Contr. Release 93(2003) 95-103;-   M. Hornof et al., In Vitro Evaluation of the Permeation Enhancing    Effect of Polycarbophil-Cystein Conjugates on the Cornea of    Rabbits, J. Pharm. Sci. 91 (12) 2002, 2588-2592;-   Clausen et al., The Role of Glutathione in the Permeation Enhancing    Effect of Thiolated Polymers, Pharm. Res. 19 (5) 2002, 602-608;-   Yamashita et al., Synthesis and Evaluation of Thiol Polymers, J.    Macromol. Sc. 26 (1989), 9, 1291-1304;-   Zheng et al., Disulfide Cross-Linked Hyaluronan Hydrogels,    Biomacromolecules 3 (6) 2002, 1304-1311;-   Wang et al., Chitosan-NAC Nanoparticles as a Vehicle for Nasal    Absorption Enhancement of Insulin, J. Biomed Mater Res Part B: Appl    Biomater 88B: 150-161, 2009;-   WO 2008/094675 A2;-   U.S. Pat. No. 5,412,076 A.

However, so far no formulation containing thiolated chitosan has beendisclosed which fulfills the requirements of long-term stability,tolerability, safety, effectiveness in the treatment of dry eye syndromeand improved patient compliance. As a result of the ineffectiveness andinconvenience of current therapies of dry eye treatment, there remains aneed for a method of treating dry eye syndrome that fulfills therequirements listed above.

SUMMARY OF THE INVENTION

The present invention is directed to an aqueous ophthalmic solutioncontaining a thiolated chitosan. More particularly, the presentinvention is directed to a sterile aqueous ophthalmic solutioncontaining about 0.05% to about 0.5% (w/w) ofN—(N-acetylcysteinyl-)chitosan or a pharmaceutically acceptable saltthereof, in a carrier solution, wherein theN—(N-acetylcysteinyl-)chitosan has a content of free thiol groups in anamount of about 80 μmol/g polymer to about 280 μmol/g polymer.

The present invention is, furthermore, directed to a containercomprising said aqueous ophthalmic solution.

The present invention is also directed to the specific use of saidaqueous ophthalmic solution in the prevention or treatment of dry eyesyndrome dry eye signs and/or symptoms. Accordingly, the presentinvention is also directed to a method of treating said diseases andsymptoms in a subject in need of such treatment. The method comprisesthe step of first identifying a subject suffering from dry eye syndromeor dry eye symptoms, then administering to the subject an effectiveamount of the aqueous ophthalmic solution containingN—(N-acetylcysteinyl-)chitosan or a pharmaceutically acceptable saltthereof.

Particularly, the present invention is suitable for treating dry eyesigns and/or symptoms caused by or connected with one or more ofkeratoconjunctivitis sicca (KCS), age-related dry eye, Stevens-Johnsonsyndrome, Sjogren's syndrome, ocular cicatrical pemphigoid, cornealinjury, infection, Riley-Day syndrome, congenital alacrima, PRK, LASEKand/or LASIK treatments, allergic conjunctivitis, blepharitis andmeibomian gland dysfunction, nutritional disorders or deficiencies(including vitamin deficiency), pharmacologic side effects, glandularand tissue destruction, autoimmune and other immunodeficient disorders,and inability to blink in comatose patients. Also included are dry eyesymptoms caused by environmental exposure to airborne particulates,smoke, smog, and excessively dry air; as well as contact lensintolerance and eye stress caused by computer work or computer gaming.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a comparison of the stability of free thiol groups in anaqueous ophthalmic solution containing 0.1% (w/w)N—(N-acetylcysteinyl-)chitosan.HCl during long term storage at 25° C.and 40% relative humidity using different packaging as described inExample 3. Grey bars depict thiol group content at different time points(starting point, 1 month, 2 months, 3 months, 6 months, 9 months, and 12months) during storage in group 1 (packaging and storage with oxygenabsorber), white bars depict thiol group stability in group 2 (packagingand storage without oxygen absorber).

FIG. 2 shows the stability of free thiol groups in an aqueous ophthalmicsolution containing 0.1% (w/w) N—(N-acetylcysteinyl-)chitosan.HCl duringshort term storage of single dose units under ambient air, humidity andtemperature conditions for 30 days as described in Example 4.

FIG. 3 shows a schematic drawing of a N—(N-acetylcysteinyl-)chitosan.HClsubstructure.

FIG. 4 shows the results of the thiol group measurements (without TCEP:grey bars, after reduction with TCEP: white bars; mean values of 3measurements with standard deviations) of polymer 1 and polymer 6 whichwere synthesized as described in example 6 and analysed as described inexample 7.

FIG. 5 shows the results of the determination of the dynamic viscosity(without TCEP: grey bars, after reduction with TCEP: white bars) of a0.5% (w/w) solution of polymer 1 and polymer 6, which were synthesizedas described in example 6 and analysed as described in example 7.

FIGS. 6, 7 and 8 show the determination of the linear viscoelastic rangeof a N—(N-acetylcysteinyl-)chitosan.HCl-mucin network.

FIG. 9 shows the calculated area under the curve values (0-96 h) derivedfrom individual time-radioactivity concentration curves of eyes treatedwith ¹²⁴I-chitosan-NAC, ¹²⁴I-chitosan.HCl and Na¹²⁴I, respectively.

FIG. 10 shows area-under the curve (AUC) values (0-72 h) calculated fromindividual time-radioactivity concentration curves (% AD*h/g) of eyestreated with ¹²⁴I-chitosan-NAC and ¹²⁴I-labeled thiolated hyaluronan(HA-cysteamine).

DETAILED DESCRIPTION OF THE INVENTION

The inventors have unexpectedly discovered a stable ophthalmic solutionthat is effective in treating dry eye syndrome or improving one or moredry eye signs and/or symptoms in patients suffering from dry eyesyndrome and/or improving the quality of the tear film in patients withdry eye disease. The present inventive sterile aqueous ophthalmicsolution contains about 0.05% to about 0.5% (w/w) ofN—(N-acetylcysteinyl-)chitosan or a pharmaceutically acceptable saltthereof, in a carrier solution, wherein theN—(N-acetylcysteinyl-)chitosan has a content of free thiol groups in anamount of about 80 μmol/g polymer to about 280 μmol/g polymer.Preferably, as the pharmaceutically acceptable salt the hydrochloridesalt is employed.

In the following, the term “chitosan-NAC” stands for bothN—(N-acetylcysteinyl-)chitosan and pharmaceutically acceptable saltsthereof.

The method of the present invention is an improvement upon the currentmost commonly used treatment of dry eye disease—artificial tears (i.e.,saline solution or solutions containing polymers) and anti-inflammatoryagents (cyclosporine).

The present invention is suitable for treating any dry eye syndrome.Broadly, dry eye syndrome or dry eye disease as pertaining to thepresent invention can be any syndrome associated with tear filminstability and/or dysfunction (such as increased tear evaporationand/or reduced aqueous secretion).

Particularly, the present invention is suitable for treating dry eyediseases caused by or connected with one or more of keratoconjunctivitissicca, age-related dry eye, Stevens-Johnson syndrome, Sjogren'ssyndrome, ocular cicatrical pemphigoid, corneal injury, infection,Riley-Day syndrome, congenital alacrima, PRK, LASEK and/or LASIKtreatments, allergic conjunctivitis, blepharitis and meibomian glanddysfunction, nutritional disorders or deficiencies (including vitamindeficiencies), pharmacologic side effects, glandular and tissuedestruction, autoimmune disorders, immuno-deficient disorders, andinability to blink in comatose patients. Also included are dry eyesymptoms caused by environmental exposure to airborne particulates,smoke, smog, and excessively dry air; as well as contact lensintolerance and eye stress caused by computer work or computer gaming.

As stated in the background, there are other diseases that have a highdegree of co-morbidity with dry eye disease: Allergic conjunctivitis(seasonal and chronic), blepharitis and Meibomian gland dysfunction.These conditions affect the quality and stability of the tear film,which results in dry eye signs and symptoms.

Laser assisted vision correction procedures such as photorefractivekeratectomy (PRK), laser-assisted sub-epithelial keratectomy (LASEK) andlaser-assisted in situ keratomileusis (LASIK) also negatively influencetear film functionality and frequently cause (temporary) dry eyedisease.

The present method of treatment comprises the steps of: (i) identifyinga subject suffering from dry eye disease; and (ii) administering to theeyes of the subject an aqueous ophthalmic solution comprising an aqueousophthalmic solution containing chitosan-NAC; wherein said dry eyedisease is caused by or connected with one or more of the diseases,syndromes and stress factors mentioned above.

One significant advantage of the formulation of the present invention isthat, following a single instillation onto the subject's eye,chitosan-NAC has a restorative effect on the tear film thickness for upto 24 h. This means that a subject does not have to constantly instillthe product in the eye as would be needed for tear replacement therapy,such as when using artificial tears. The daily topical dose of theaqueous ophthalmic chitosan-NAC solution, effective to reduce dry eyesymptoms and/or to improve tear film composition can be divided amongone or several unit dose administrations. A subject would use theproduct as needed, but generally this would not be more than twice a dayand in many instances the product would be used only once a day. Apreferred regimen for the chitosan-NAC solution of the present inventionis one drop of 0.1% (w/w) solution per eye once a day. Thus, asignificant advantage of the formulation of the present invention isthat it could be a once a day product and would ensure better patientcompliance.

After treatment by the present methods, one or more dry eye signs and/orsymptoms are reduced or alleviated in the subject. Dry eye symptomsinclude dryness, burning, ocular itching, ocular discomfort,photophobia, foreign body sensation, blurry vision, grittiness, andvisual disturbance and/or loss, including blurred vision, reducedreading speed, photophobia, and loss in visual acuity. Dry eye signs areassessed by measurements such as: corneal and/or conjunctival staining(using fluorescein, lissamine green or rose Bengal stain), Schirmer'sstrip testing, Zone-Quick threads, tear film osmolarity, tearbreak-up-time and tear meniscus height.

One reason the signs and/or symptoms of DES are improved using theformulation of the present invention is possibly the restorative effectof the formulation on the ocular tear film thickness, which may becaused by a chemical interaction of the thiolated chitosan with ocularsurface mucins or the presence of a protective coating on the ocularsurface which is formed by crosslinking of chitosan-NAC afterapplication to the ocular surface, or a combination of both effects.

The “effective amount” of the aqueous ophthalmic solution containing thechitosan-NAC polymer administered to a subject is an amount effective toreduce the clinical signs and/or symptoms of dry eye disease. Thisamount comprises about 0.05% to about 0.5% (w/w) of chitosan-NACpolymer, in a carrier solution.

At an amount below 0.05% the chitosan-NAC is in such a low concentrationthat no clinical effect would be seen in a patient. At much over 0.5%the patient tolerance starts to suffer and any permeation enhancingeffects of chitosan are more pronounced. The concentration of thechitosan-NAC in the aqueous ophthalmic solution is preferably 0.05 to0.3% (w/w), more preferably 0.05 to 0.2% (w/w), most preferred0.08-0.16% (w/w). At a concentration much below 0.05% the mucoadhesiveproperties and consequently the clinical effect are not as pronounced.At a concentration much over 0.3% the high viscosity of the chitosan-NACformulation leads to the formation of “clumps” after ocular applicationand the solution is not as evenly spread out on the ocular surface.Solutions of chitosan-NAC in a concentration range of 0.05% to 0.3% areevenly distributed on the ocular surface as demonstrated by in vivostudies with Iodine-¹²⁴ radiolabeled chitosan-NAC (see examples 9 and10). The concentration of chitosan-NAC in the aqueous ophthalmicsolution is most preferably about 0.1% as both clinical effect andpatient tolerance are optimal.

A large percentage of the thiol groups bound on the chitosan-NAC thatare present in the aqueous ophthalmic solution needs to be in the formof free thiol groups, that is to say, non-crosslinked thiols as definedbelow. It is important that the majority of these thiol groups remainfree so as to only react upon installation onto the ocular surface. Insome instances the amount of free thiol groups could be fairly low andstill provide some pharmaceutical benefit to the eye when it isinstilled. However, it is preferable that the chitosan-NAC polymer inthe formulation of the present invention has a content of free thiolgroups in an amount from about 80 μmol/g polymer to 280 μmol/g polymer;preferably with a range of free thiol groups of about 105 μmol/g polymerto 250 μmol/g polymer, more preferably of from 110 μmol/g polymer to 250μmol/g polymer and most preferably of from preferably 140 to 250 μmol/gpolymer.

At a free thiol group concentration much below 80 μmol/g polymerinteractions between chitosan-NAC and mucins are weak and comparable tounmodified chitosan HCl, however at very high concentrations of freethiol groups of 300 μmol/g and more, the physicochemical properties ofthe core polymer chitosan are changed to such an extent that the polymeris not useful for preparation of an aqueous ophthalmic solution due toits reduced cationic character and limited solubility in water.

This amount of free thiol groups immobilised on the chitosan-NAC needsto be present when the ophthalmic formulation of the present inventionis ready to be used in the eyes of the dry eye patient. This means thatthe free thiol groups present in the aqueous ophthalmic solution of thepresent invention must survive relatively long storage times. Therefore,the resulting dry eye product must be stable and retain the above statedfree thiol groups for at least about 12 months, more preferably at leastabout 18 months, and even more preferably at least about 24 months.

The amount of free thiol groups immobilised on chitosan-NAC in anaqueous ophthalmic solution can be determined by the skilled artisan ina known way, such as via Ellman's reagent (see example 1).

In addition to the fact that a high amount of free thiol groups on thechitosan-NAC polymer in the aqueous ophthalmic solution is important, alow amount of crosslinked thiols (disulfides) on the chitosan-NACpolymer in the solution of the present invention is also preferable.During preparation and storage of the aqueous ophthalmic solutioncrosslinking of thiol groups immobilised on the chitosan-NAC can occur.A low amount of crosslinked thiols present in the formulation is apreferred parameter of the chitosan-NAC polymer formulation of thepresent invention.

Therefore, according to a preferred embodiment of the present invention,the amount of crosslinked thiol groups in theN—(N-acetylcysteinyl-)chitosan is 30% or less of the total thiol groupstherein, preferably 25% or less, most preferably 15% or less.

Especially, in this preferred embodiment, the amount of crosslinkedthiol groups in the N—(N-acetylcysteinyl-)chitosan is 30% or less of thetotal thiol groups therein, preferably 25% or less, most preferably 20%or less after storage of the solution for at least 12 months at roomtemperature.

If the amount of crosslinked thiol groups present in the formula was toohigh, the properties of the aqueous ophthalmic solution could changeoutside of the desired parameters, for example, the viscosity of theaqueous ophthalmic solution could become too high to be suitable for eyedrops.

As explained below in more detail, it has been found that it is possibleto produce a chitosan-NAC the thiol groups of which are not or onlyminimally crosslinked, such as with an amount of crosslinked thiolgroups of less than 5%, preferably 4% or less of the total thiol groups.Especially if such a chitosan-NAC is employed for the manufacture of theophthalmic solution of the present invention, the free thiol groups tendto be stable during the entire life cycle of the solution:

Thus, it has been found that upon employing such a chitosan-NAC duringproduction of the formulation the increase of crosslinked thiol groupsis <10% of the amount of free thiol groups initially present on thechitosan-NAC raw material. Furthermore, during storage of the solutionover 12 months or even 18 months the increase of crosslinked thiolgroups is <15% of the amount of free thiol groups initially present inthe formulation. Finally, even if a second container of the solution (asdefined below) which provides an oxygen barrier is opened, 30 days afteropening the increase of crosslinked thiol groups is <15% of the amountof free thiol groups initially present in the formulation beforeopening.

Essentially the thiolated chitosan ophthalmic formulation of the presentinvention is made according to the following steps:

-   -   1. Chitin is isolated from crustaceous shells, such as shrimp or        snow crab shells;    -   2. Chitosan is prepared from chitin through a chemical process        that is well known in the art as, for example alkaline        deacetylation;    -   3. The chitosan is thiolated by the covalent attachment of a        thiol bearing ligand, such as with the use of N-acetylcysteine        as is set forth herein;    -   4. The chitosan-NAC is then formulated in the form of an aqueous        ophthalmic solution as is set forth herein; and    -   5. The aqueous ophthalmic solution containing chitosan-NAC is        then put into a suitable container that would ensure its        stability as is set forth herein.

As described in detail herein, chemically, chitosan is a polycationicbiopolymer with favourable biological properties such as highbiocompatibility and low toxicity. The source of the raw materials toproduce the chitosan for the formulation of the present invention is notcritical as long as the resulting chitosan is fairly pure and free fromcontaminants. For example, chitosan can also be produced from anon-animal source. The chitosan of the present invention can beprocessed by various methods so long as the chitosan has the propertiesrequired and set forth in the present specification. Most commonly, theraw material for the chitosan comes from chitin in the shells ofcrustaceans. Chitin can be obtained during the harvesting of crustaceousfood sources such as lobster, crab, and shrimp.

One preferred raw material source of chitin for the chitosan processedand used in the formulation according to the present invention is fromthe snow crab (Chionoecetes opilio) shells. Various sources ofcrustaceans could be used in obtaining the chitin to make the chitosanas long as the supply would be adequate. The preparation of chitosanfrom raw material biological sources is well known and disclosed innumerous scientific publications, for example (Chang, Tsai et al., 1997,Heterogeneous N-deacetylation of chitin in alkaline solution, CarbohydrRes (303): 327-332; Dutta, Dutta et al., 2004, Chitin and chitosan:Chemistry, properties and applications, J Sci Ind Res (63): 20-31).

The chitosan-NAC can be synthesized by reacting the chitosan with athiol-group bearing ligand. Methods for thiolation are disclosed innumerous scientific publications, for example (Seki, Aoi et al., 1999,Partially deacetylated chitin or chitosan derivative with selectivelysubstituted free amino group by amino acid and peptide and havingsubstantially no substituted hydroxide group, and its manufacture,JP2000-302803; Bernkop-Schnurch, Hornof et al., 2004, Thiolatedchitosans, Eur J Pharm Biopharm (57): 9-17; Schmitz, Grabovac et al.,2008, Synthesis and characterization of a chitosan-N-acetyl cysteineconjugate, Int J Pharm (347): 79-85; Zhu, Su et al., 2012, Synthesis ofthiolated chitosan and preparation nanoparticles with sodium alginatefor ocular drug delivery, Mol Vis (18): 1973-1982).

The chitosan-NAC used in the present invention needs to be water solublein the concentration range useful for the preparation of the aqueousophthalmic solution and the resulting solutions need to be clear andcolorless. Salt formation of chitosan-NAC with organic or anorganicacids increases the aqueous solubility of chitosan. Suitable salts ofthiolated chitosan of the present invention include any pharmaceuticallyacceptable salts with organic acids such as acetic, citric, formic andtartaric acid, as well as mineral acids such as HCl and H₂SO₄. Thechitosan-NAC hydrochloride salt is a preferred embodiment of the presentinvention.

What is important is that such reaction pathways and reaction conditionsare used that after synthesis and purification essentially all thiolgroups immobilised on the chitosan backbone are present in the free formand not in the crosslinked form as disulfides, i.e. are only minimallycrosslinked. Virtually all attached thiols in the thiolated chitosan ofthe present invention are in the form of free thiol groups, i.e. theyare not cross-linked. A minimal amount of crosslinking during synthesisis only acceptable as long as the viscosity of the thiolated chitosanremains within the stated parameters and its aqueous solubility issufficient for the preparation of an aqueous ophthalmic solution.

It has been found that it is possible to manufacture chitosan-NAC with avery low or even zero degree of crosslinking of the thiol groups byexposing the chitosan-NAC to a reducing agent after its synthesis, forexample after alkaline hydrolysis of the thioacetyl moieties. Thereducing agent may be selected from the group of DTT, TCEP or NaBH₄,NaBH₄ being preferred. It has, furthermore, been found that thereduction step should be carried out at elevated temperature, such as30° C. or more or preferably 40° C. or more. Furthermore, high amountsof reducing agents need to be employed, such as with a stoichiometricratio of reducing agent to the chitosan backbone polymer of 2:1 or more.

Chitosan-NAC polymers with a degree of crosslinked thiol groups of lessthan 5%, preferably 4% or less of the total thiol groups can besynthesized according to this embodiment.

The viscosity in aqueous solution of the final chitosan-NAC of thepresent invention preferably falls within a certain range, and thepresent inventors have unexpectedly discovered that the viscosity of thechitosan-NAC only falls within this preferred range if during theproduction of the chitosan-NAC the chitosan-NAC is processed undercertain conditions and within certain parameters, particularly accordingto the reduction conditions stated above, which lead to polymers whichare only minimally crosslinked. The viscosity of the resulting productpreferably falls within an acceptable range so that the chitosan-NACwill be most useful in the resulting eye drop formulation. Thus, thekinematic viscosity (0.5% in water at 25° C.) of the chitosan-NACpolymer is preferably within the range of about 1 to 15 mm²/s, morepreferably within the range of about 2 to 10 mm²/s. If the viscosity istoo high, then a useful eye drop solution cannot be made with thepreferred concentration range of chitosan-NAC in the formulation, as thepolymer will remain as an insoluble viscous mass in the container.

The chitosan-NAC needs to be purified to be useful in the formulation ofthe present invention (such as after step #3 above and, especially,after treatment of the chitosan-NAC with the reducing agent). Thechitosan-NAC should be washed in such a way that the resulting productis pure. One known method is disclosed in Kast and Bernkop-Schnurch,2001, Thiolated polymers—thiomers: development and in vitro evaluationof chitosan-thioglycolic acid conjugates, Biomaterials (22): 2345-2352.

Another method would be washing the chitosan-NAC with polar solventsfollowed by drying in order to remove the solvents. One preferredsolvent is isopropyl alcohol, since it is non-toxic, readily available,and economical, however other solvents, and other alcohols other thanisopropyl alcohol could work as well. This washing can be repeated asneeded, depending upon the volume of solvent used each time. Preferablythe washing and drying step is repeated at least one time.

The drying step can be conducted at room temperature and at standardhumidity, but this process can be very time consuming. Therefore, thedrying process is preferably conducted at an elevated temperature and/orunder reduced pressure. The drying of the chitosan-NAC is preferablyconducted at an elevated temperature of at least about 40° C. to about70° C. and preferably for at least about five hours. A more preferreddrying process is conducted at temperatures of at least about 50° C. toabout 60° C. for about 10 to 24 hours. One preferred multi-steppurification process would be to wash the chitosan-NAC polymer threetimes with isopropyl alcohol and to recover the solid by centrifugationfollowed by drying at about 60° C. for about 15 to 20 hours.

The aqueous ophthalmic solution according to the present invention cancontain at least one ophthalmic compatible excipient. Any excipientsuitable for example to adjust the tonicity, the viscosity of thesolution or to stabilise the pH, to increase the solubility of theactive ingredient, to increase ocular comfort after application, or tostabilise the formulation in general, can be employed.

The pH of the aqueous ophthalmic solution is adjusted by adding anyphysiologically and ophthalmic acceptable pH adjusting acids, bases, orbuffers to have a pH within the range of about 5.5 to about 7. A pH muchbelow about 5.5 would be outside of the physiological acceptableparameters (the solution would cause a severe stinging or burningsensation in the eye). At a pH much above 7, forming a stable solutionof the chitosan-NAC where it does not precipitate out of solution isdifficult. Thus, due to the ease of formulating a stable solution, a pHbelow 7 is preferred. The preferred pH of the aqueous ophthalmicsolution of the present invention is between about 5.8 to about 6.8,with a pH of 6.0 to 6.6 being most preferred.

Examples of suitable acids used in the formulation of the presentinvention include acetic, boric, citric, lactic, phosphoric,hydrochloric, and the like, and examples of bases include sodiumhydroxide, sodium phosphate, sodium borate, sodium citrate, sodiumacetate, sodium lactate, tromethamine, THAM(trishydroxymethylamino-methane), and the like. A preferred embodimentof the present invention is a boric acid and sodium borate buffersystem, which also contains a polyol such as mannitol to increase buffercapacity at the most preferred pH range of 6.0 to 6.6.

Examples of suitable excipients used in the formulation of the presentinvention to increase stability of the formulation include disodiumethylenediaminetetraacetate (Na2-EDTA), sodium metabisulfite, mannitol,polyethylene glycol and the like.

The osmolarity of the topical ophthalmic formulation of the presentinvention is generally from about 150 to about 400 milliosmolar (mOsM),more preferably from about 200 to about 350 mOsM, with the osmolarity ofabout 250 to about 330 mOsM being most preferred. The osmolarity can beadjusted by using appropriate amounts of physiologically and ophthalmicacceptable ionic or non-ionic agents. Sodium chloride is a commonosmotic agent. Equivalent amounts of one or more salts made up ofcations such as potassium, ammonium and the like and anions such aschloride, citrate, ascorbate, borate, phosphate, bicarbonate, sulfate,thiosulfate, bisulfate, and the like can be used in addition to orinstead of sodium chloride to achieve osmorality within the above-statedrange. Further, non-ionic agents such as mannitol, dextrose, sorbitol,glycerol, glucose and the like can also be used to adjust theosmolarity. Sodium chloride and mannitol are the preferred agents toadjust osmotic pressure.

The ophthalmic formulation can contain lubricants to provide a highocular comfort level suitable for the regular application necessary inthe treatment of DES. There are many types of lubricating agents such aspolyvinylpyrrolidone, polyvinylalcohol, liquid polyols, hyaluronic acidand pharmaceutically acceptable salts thereof, lubricin and cellulosederivatives; however preferred agents are polyethylene glycol andhydroxypropyl methylcellulose (HPMC).

In a preferred embodiment, the ophthalmic solution according to thepresent invention contains the following excipients in addition toN—(N-acetylcysteinyl-)chitosan hydrochloride:

-   -   Boric acid in an amount from 1.0 to 16.0 mg/ml, preferably 8 to        16 mg/ml;    -   Polyethylenglycol 400 in an amount from 0.01 to 5.0 mg/ml,        preferably 1 to 5 mg/ml;    -   Naz-EDTA in an amount from 0.01 to 0.5 mg/ml;    -   Mannitol in an amount from 0.01 to 5.5 mg/ml, preferably 0.1 to        4 mg/ml;    -   Sodium chloride in an amount from 0.01 to 9 mg/ml, preferably 1        to 3 mg/ml; and    -   Hydroxypropyl methylcellulose in an amount from 0.01 to 20        mg/ml, preferably 1 to 3 mg/ml.

The ophthalmic solution according to the present invention has to besterile and can be sterilized in any suitable manner. One particularpreferred sterilization method is sterile filtration. The ophthalmicsolution according to the present invention can contain preservatives,such as benzalkonium chloride, although this is less preferred.

The aqueous ophthalmic solution containing chitosan-NAC can beadministered to the eyes of a patient by any suitable means for topicaladministration. This is preferably in the form of an aqueous eye dropsolution. This solution can be in a single use container that is sterileuntil opened and thus does not need to have a preservative, or it can bein the form of a multi-use container that remains sterile after openingor in a multi-use container with a formulation containing apreservative.

The thiol groups of chitosan-NAC polymers tend to form disulfide bondsin aqueous solutions, thus reducing the mucoadhesive properties ofchitosan-NAC. It was discovered that this tendency depends on thepresence of oxygen in the aqueous ophthalmic solution.

It has been found that it is possible to stabilize the free thiol groupsof the chitosan-NAC employed according to the present invention inaqueous solution even more when storing the solution under oxygen-freeconditions, or essentially oxygen-free conditions. The oxygen-freeatmosphere can be a nitrogen atmosphere, vacuum atmosphere, or anatmosphere consisting of noble gases.

Thus, when the solution is put into a container it should be done so inthe absence of oxygen. Further, after the container is filled with theaqueous ophthalmic solution of the present invention, it should remainoxygen free. Therefore, the present invention also relates to acontainer that keeps the aqueous ophthalmic solution free from oxygenduring storage. Accordingly, one aspect of the present inventioncomprises an essentially oxygen free container containing the aqueousophthalmic solution. As “essentially oxygen free”, an atmosphere with anamount of 1.5% oxygen or less is to be understood. The concentration ofdissolved oxygen in solution during production of the formulation andfilling into the containers is below 1.0 mg/L, more preferably below 0.5mg/L, even more preferably in the range of 0.1 mg/L.

In a preferred embodiment, the container is made of a material that isimpervious to oxygen such that after filling, the ophthalmic solutionremains essentially oxygen free for an extended period of time. Suchcontainers could be glass or glass lined polymers, metal or metal linedpolymers. In another preferred embodiment, the container is made of apolymer that has contained therein an oxygen absorber that would preventoxygen from entering the solution through the walls of the container.Such oxygen absorbers include iron salts, sulfites, ascorbic acid,unsaturated fatty acid salts, metal-polyamide complexes or palladium/H₂based systems. For example, WO 09/32526 discloses a film having anactive oxygen barrier layer comprising an oxygen scavenging compositionblended of a thermoplastic resin having carbon-carbon double bondssubstantially in its main chain, a transition metal salt, and an oxygenbarrier polymer with oxygen barrier properties.

Further, the container itself can be manufactured from a gas tightmaterial with an oxygen scavenger embedded and an airless closuresystem.

In a preferred embodiment, there is provided a first containercontaining the ophthalmic solution and a second container containingsaid first container.

Thus, for example, the container that holds the ophthalmic solution ofthe present invention is itself contained inside of a gas tight sachetor pouch. In particular a sachet or pouch made of aluminium or analuminium laminate or aluminium composition may contain therein one ormore sub-containers (i.e. “first containers”) containing the ophthalmicsolution according to the invention. The second container, i.e. thesachet or pouch can also contain an additional oxygen absorber (forexample PKT KH-20 Pharmakeep® or Stabilox® Oxygen Scavenger) as is usedin some standard packaging. Even in the case where the sachet is sealedunder vacuum or in an inert atmosphere, the addition of an oxygenabsorber can be required in order to remove residual oxygen from thesub-container. The sachet can contain either one or more single dosecontainers or multi-dose containers, for example five single dosecontainers per sachet. In the case of the multi-dose container, it mustpreserve the ophthalmic solution according to the present invention in asterile condition and in an essentially oxygen free condition.

The chitosan-NAC contained in the container according to the inventionpreferably has a content of free thiol groups of from 80 μmol/g polymerto 250 μmol/g polymer, preferably 105 μmol/g polymer to 250 μmol/gpolymer after storage of at least 12 months at room temperature. Thismeans that, according to the present invention, the free thiol groupsremain on the chitosan-NAC and that the resulting formulation is stabileover an extended period of time. This period of time is preferably atleast about 12 months, more preferably at least 18 months, and even morepreferably at least about 24 months. This long stability preference isdue to the fact that some products end up having long storage times anddelays in commercial delivery and supply chains that could result in aless stable product falling out of acceptable parameters.

Furthermore, preferably the amount of crosslinked thiol groups in thechitosan-NAC contained in the container according to the invention is30% or less of the total thiol groups therein, preferably 25% or less,most preferred 20% or less after being stored for at least 12 months,more preferably at least 18 months. As mentioned above, the stability ofthe free thiol groups in the solution is especially good if achitosan-NAC with only a minimal degree of crosslinked thiol groups isemployed for manufacturing the solution.

In the above-described embodiment where there is a second container,e.g. a gas tight sachet, containing one or more first container(s), e.g.single use containers made from LDPE, the content of free thiol groupsin the solution preferably remains within the range as defined per thepresent invention after opening of the first container for at least 30days. The therapeutic time needed for e.g. 5 containers is 5 days, thusthis duration of stability is more than sufficient.

As mentioned above, it was found that especially if a chitosan-NAC withonly a minimal degree of crosslinked thiol groups is employed formanufacturing the ophthalmic solution of the present invention, the freethiol groups remained stable even after the second container whichprovides the oxygen barrier was opened, i.e. it was found that 30 daysafter opening of the second container the increase of crosslinked thiolgroups was <15% of the amount of thiol groups initially present in thesolution before opening.

The present invention is concerned primarily with the treatment of humansubjects, but can also be employed for the treatment of other mammaliansubjects, such as dogs and cats, for veterinary purposes.

The invention is illustrated further by the following examples, whichare not to be construed as limiting the invention to the specificprocedures described therein.

EXAMPLES Example 1: Determination of Thiol Group Content

Quantification of thiol groups is based on a wet chemistry methodemploying 5,5′-Dithio-bis(2-nitrobenzoic acid, DTNB, “Ellman'sreagent”). Free thiol groups react with the reagent under formation of amixed disulfide, whereas one equivalent of 2-Nitro-5-thiobenzoic acid isset free. In buffered alkaline medium (pH=8.05), the resulting thiolategives a distinctly yellow coloured solution, whose absorption can bemeasured photometrically at 450 nm.

119.0-121.0 mg of the sample solution containing 0.1%N—(N-acetylcysteinyl-)chitosan hydrochloride were weighed in 1.5 mlEppendorf vessels. Three values were determined from each solution. 25.0mg N-acetylcysteine was accurately weighted and solved in 25.0 ml ofborate buffer (pH 6.75). This solution was then further diluted for acalibration curve. Borate puffer was used for the blank value. To eachsolution phosphate buffer pH 8.0 was added to a final volume of 600 μl.600 μl freshly prepared Ellman's solution (containing 20 mg of5,5′-Dithio-bis(2-Nitrobenzoic acid) in 100 ml of phosphate buffer (pH8.0)) was added to each solution (standard solution, sample solution andblank value). The solutions were well-mixed and incubated for 1.5 hunder UV protection. Afterwards the sample solutions containingN—(N-acetylcysteinyl-)chitosan hydrochloride were centrifuged at 13400rpm 5 minutes. Finally 1.1 ml of each solution (standard solution,sample solution and blank value) was transferred into asemimicrocuvettes and tested at 450 nm in a spectrophotometer againstthe blank value.

The mean content of free thiol groups of the thiolated chitosan in thesample solutions was determined to be 2.61% (w/v) which corresponds to avalue of 178.5 μM free thiol groups/g polymer.

Example 2: Composition of the Formulation

The following aqueous ophthalmic solution was prepared usingN—(N-acetylcysteinyl-)chitosan hydrochloride with a content of freethiol groups of 158 μM/g polymer and a kinematic viscosity (25° C., 0.5%aqueous solution) of 5.63 mm²/s. All other ingredients arepharmaceutical additives. The solution was sterilized via sterilefiltration.

The eye drop formulation had the following composition:

TABLE 1 Composition of the aqueous ophthalmic solution Compound Amount[mg] N-(N-acetylcysteinyl-)chitosan hydrochloride 1.0 Boric acid 12.4Polyethylenglycol 400 (PEG 400) 3.0 Na₂-EDTA 0.42 Mannitol 2.0 NaCl 2.0HPMC 1.5 NaOH q.s. Aqua bidestillata q.s. ad 1 ml

The physical-chemical properties of the formulation were within thefollowing specifications:

TABLE 2 Physical-chemical properties of the aqueous ophthalmic solutionpH-value 6.0-6.6 Osmolality [mOsmol/kg] 250-330 Content of free thiolgroups [μMol/g 140-250 polymer] Content of thiolated chitosan [mg/ml]0.90-1.10 Sterility sterile

This example shows that chitosan-NAC may be formulated according to thepresent invention to fulfil the requirements for ophthalmicpreparations.

Example 3: Long Term Stability of the Aqueous Ophthalmic Solution

Group 1:

A formulation according to the present invention containing 1 mgN—(N-acetylcysteinyl-)chitosan hydrochloride/ml was prepared under inertconditions and subsequently filled aseptically using blow-fill-sealtechnology into cards of five single dose containers made of LDPE (lowdensity polyethylene) to a volume of 0.3 ml. Each card was packaged inan aluminium sachet containing an oxygen absorber (PKT KH-20Pharmakeep®).

Group 2:

A formulation according to the present invention containing 1 mgN—(N-acetylcysteinyl-)chitosan hydrochloride/ml was prepared, filled incards of five single dose containers and packaged in aluminium sachetsas described above, but in this group the aluminium sachets did notcontain oxygen absorbers.

Samples of formulations prepared under manufacturing conditions forgroup 1 and group 2 were stored under controlled conditions at atemperature of 25° C. and 40% relative humidity for 18 months. Thecontent of free thiol groups was measured as described in example 1 inregular time intervals. At the same time the concentration of oxygeninside the pouch was measured with an oxygen gas analyzer (PBI DansensorCheckPoint II). The results of the thiol group measurements are shown inFIG. 1 (group 1: grey bars, group 2: white bars). The results indicatethat when the aqueous ophthalmic formulation of the present inventioncontaining chitosan-NAC polymer is stored in an essentially oxygen freeatmosphere with maximum oxygen levels of less than 1.5% (as provided byan oxygen absorber) the free thiol content remains high, withinacceptable parameters for a time period of at least 18 months.Surprisingly, even when stored without an oxygen absorber the thiolgroup content of the solution remained stable for a time period of about1 month. After 4 months the thiol group content still remained withinacceptable parameters but a decrease of thiol group content of 20% wasobserved. Oxygen levels in the pouches of group 2 were in the range of3.0 to 5.7% during the entire storage period of 18 months.

Example 4: Short Term Stability of the Aqueous Ophthalmic Solution afterRemoval of Single Dose Containers from Aluminium Pouch

A formulation according to the present invention containing 1 mgN—(N-acetylcysteinyl-)chitosan hydrochloride/ml was prepared under inertconditions and subsequently filled aseptically into cards of five singledose containers made of LDPE (low density polyethylene) to a volume of0.3 ml using blow-fill-seal technology. Each card was packaged in analuminium sachet containing an oxygen absorber (PKT KH-20 Pharmakeep®).Within one month after production the cards were removed from thepouches and stored in closed folding boxes under ambient air, humidityand temperature conditions for 30 days. The content of free thiol groupsin the formulation was measured on day 0, day 5, day 12, day 19 and day30.

The results are shown in FIG. 2 (mean values of 3 batches with standarddeviations). These results indicate that the product remains within thedesired parameters for a time period of 30 days when stored in LDPEsingle dose units in the presence of oxygen under ambient airconditions. The decrease of free thiol groups after 30 days was about10%. Similar results were observed even when the aluminium sachetcontaining the cards were stored for 5 months at a temperature of 40° C.before removing the cards.

Example 5: Synthesis of N—(N-Acetylcysteinyl-)Chitosan Hydrochloride

N—(N-acetylcysteinyl-)chitosan hydrochloride polymers (FIG. 3) withdifferent degrees of thiolation were synthesized by covalent attachmentof N,S-diacetylcysteine to the primary amines of chitosan vianucleophilic substitution followed by release of free thiol groups inalkaline media. Different chitosans were used as raw materials for thesynthesis (see Table 3). Data on the source and degree of deacetylationwere provided by the supplier. Molecular weight was determined via sizeexclusion chromatography (SEC)-coupled triple detection using theViscotek TDA305 from Malvern Instruments. The samples were dissolved inacetic acid 5% and separated isocratically on ViscoGEL columns.

TABLE 3 List of chitosan raw material specifications Raw material Degreeof (supplier) Source deacetylation Molecular weight Chitosan (Messrs.Pandalus borealis   85-95%  25 kDa-250 kDa Primex) Chitosan (Messrs.Chionoecetes 89%-93% 100 kDa-250 kDa HMC⁺) opilio Chitosan HClChionoecetes 88%  20 kDa (Messrs. HMC⁺) opilio Chitosan HCl Unspecified91% 150 kDa (Messrs. Aoxing) animal source

The resulting degree of thiolation was dependent on the weight ratio ofchitosan and the active ester of N'S-diacetylcysteine as well as on thereaction conditions (pH value and temperature during nucleophilicsubstitution). Chitosan-NAC polymers with a degree of thiolization inthe range of 40 μM/g polymer to 400 μM/g polymer were synthesized.

For the synthesis of the 5 different batches of chitosan-NAC polymerlisted in Table 4 the following reaction conditions were employed:Chitosan with a molecular weight of 100 kDa to 250 kDa (supplier HMC+)was used as starting material. The weight ratio between chitosan and theactive ester of N'S-diacetylcysteine was 1:1 (wt %) and the weight ratiobetween chitosan and the reducing agent sodium borohydride was 1:2 (wt%). The reduction with sodium borohydride was carried out at an elevatedtemperature of about 40° C.

TABLE 4 Characterisation of chitosan-NAC polymers in terms of molecularweight and degree of thiolization Molecular weight Degree ofthiolization 145 kDa 220 μM/g polymer 165 kDa 225 μM/g polymer 200 kDa210 μM/g polymer 235 kDa 220 μM/g polymer 200 kDa 220 μM/g polymer

All chitosan-NAC polymers listed in Table 4 were suitable for thepreparation of an aqueous ophthalmic solution according to the presentinvention in terms of water solubility, viscosity and thiol groupcontent.

Example 6: Reaction Conditions for the Synthesis of MinimallyCrosslinked Chitosan-NAC

The covalent attachment of N,S-diacetylcysteine to the primary amines ofchitosan was followed by alkaline hydrolysis of the thioacetyl-moitiesat pH 8±0.2. Resulting intra- and intermolecular disulfide bonds werethen reduced under inert conditions at 40° C. using a large excess ofsodium borohydride (chitosan:sodium borohydride=1:2 (wt.-%)).Subsequently, excess of sodium borohydride was destroyed by addition of5N HCl which simultaneously generated the chitosan-NAC hydrochloridesalt at pH 1±0.1. The off-white product was precipitated with2-propanol, recovered by centrifugation and dried according to a definedprocedure.

As shown in numerous experiments, elevated temperatures as well assurprisingly high amounts of reducing reagent were essential for aquantitative and reproducible reduction as disulfide crosslinkedchitosan-NAC polymers form hydrogels or become insoluble in aqueoussystems (see Table 5).

TABLE 5 Reducing conditions Aqueous solution of Chitosan-NAC TemperatureChitosan:NaBH₄ [wt.-%] low viscosity polymer No. 20° C. 1:1 NO 1 1:2 NO2 30° C. 1:1 NO 3 1:2 NO 4 40° C. 1:1 NO 5 1:2 YES 6

The only option to potentially utilize Polymers 1-5 for the manufactureof the aqueous ophthalmic solution according to this invention was tothermally degrade these disulfide crosslinked derivatives at 60° C. formore than 4 days (depending on the degree and type (inter- andintramolecular) of cross-linking) to meet the specifications in terms ofkinematic viscosity of a 0.5 wt.-% aqueous solution, which is preferablyin the range of 1 to 15 mm²/s. In addition, the drying process is verytime consuming. In contrast, the minimally crosslinked chitosan-NACpolymer No. 6 synthesized under optimized reaction conditions fellwithin the preferred range of the viscosity after a drying time of lessthan 20 hours at 60° C.

Example 7: Characterisation of Minimally Crosslinked Chitosan-NAC

Chitosan-NAC polymers Nos. 1 and 6 according to example 6 werecharacterised in terms of relative content of free thiol groups,molecular weight and dynamic viscosity before and after selective andquantitative reduction of any disulfides present in the final polymerswith tris(2-carboxyethyl)phosphine (TCEP).

Determination of relative thiol group content: The Chitosan-NAC polymerssolubilized as aqueous solutions of 1.5% (w/w) were incubatedwith/without TCEP (3 mg/ml) for 90 minutes at room temperature. Afteracidification with 1N HCl, the Chitosan-NAC polymers were repeatedlyprecipitated with 2-propanol and recovered by centrifugation. Theresidue was dissolved in distilled water followed by addition of2,2′-Dithiodipyridine which generates a stable thione upon reaction withfree thiol moieties. This tautomeric substance could be easilyquantified by UV analysis at 343 nm.

Measurement of dynamic viscosity: The Chitosan-NAC polymers solubilizedas aqueous solutions of 0.5% (w/w) were incubated with/without TCEP (2mg/ml) for 90 minutes at room temperature. Rheological characterizationwas done via rotational measurement at constant shear rate of 5 s⁻¹ at25° C.

Molecular weight measurement (SEC analysis): In order to determine theinfluence of the reaction conditions on the molecular weight of thechitosan backbone, an unmodified chitosan HCl salt was prepared by usingthe identical reaction conditions for the synthesis of polymer 6 asdescribed in example 6 but without the addition of the active ester ofN'S-diacetylcysteine. The chitosan and Chitosan-NAC polymers solubilizedin acetic acid 5% in a concentration of 0.1% (w/w) were incubatedwith/without TCEP (1 mg/ml) for 90 minutes at room temperature.Molecular weights were separated isocratically at a flow rate of 0.7ml/min and determined by triple detection using the Viscotek TDA305 fromMalvern Instruments.

The results of the thiol group measurements of polymer 1 and polymer 6are shown in FIG. 4 (without TCEP: grey bars, after reduction with TCEP:white bars; mean values of 3 measurements with standard deviations). Theresults of the determination of dynamic viscosity of a 0.5% (w/w)solution of polymer 1 and polymer 6, respectively, are shown in FIG. 5(without TCEP: grey bars, after reduction with TCEP: white bars).

These results indicate that even the presence of a relatively smallamount of disulfide crosslinked structures (9% of all N-acetylcysteinegroups immobilized on the polymer in the case of polymer 1, without TCEPtreatment) had a massive impact on the viscosity of chitosan-NAC polymerin an aqueous solution, whereas the minimally cross-linked chitosan-NACwith only about 3% of its thiol groups present as disulfides solubilisedas aqueous solution of low viscosity which was suitable for thepreparation of eye drops.

Additionally, the correlation between rheological properties of thepolymers and the content of disulfide crosslinking was confirmed by SECanalysis. The minimally cross-linked chitosan-NAC polymers (polymer 6)showed a slight decrease of the molecular weight of 20-30% afterquantitative reduction by TCEP. After quantitative reduction themolecular weight of polymer 6 was comparable to the molecular weight ofthe unmodified chitosan.HCl. The main amount of the partiallycross-linked (9%) chitosan-NAC (polymer 1) however exhibited molecularweights beyond the exclusion limits of the columns and even theanalyzable fraction (about 20%) yielded molecular weights four times ofits initial weight. After quantitative reduction with TCEP the molecularweight of polymer 1 was in the same range as the molecular weight of theunmodified chitosan HCl. As expected, reaction of unmodifiedchitosan.HCl with TCEP did not result in any molecular weight decrease.

Example 8: Mucoadhesion Testing

The mucoadhesive properties of chitosan-NAC polymers with differentdegrees of free thiol groups were evaluated by measuring theirinteraction with isolated mucins. Chitosan-NAC polymers were synthesizedas described in example 5 and used to prepare aqueous ophthalmicformulations as described in example 2. Unmodified chitosan.HCl was usedas control.

Aqueous ophthalmic formulations containing chitosan-NAC or chitosan.HCl,respectively, were incubated at 32° C. for 30 min with aqueous solutionsof isolated and cesium chloride gradient purified pig gastric mucins.The final concentration of polymer in each sample was 0.15% (w/w).Aggregates were allowed to form over night and were removed from thesolution via centrifugation on the next day. After centrifugation theaggregates were resuspended in distilled water for the rheologicalmeasurements. The stability of the resulting suspensions was evaluatedvia oscillatory amplitude sweep tests (Rheometer MCR 101, cone-platemeasuring system) at an angular frequency of 10 rad/s to determine thelinear viscoelastic range.

Results of this example showed that chitosan-NAC polymers with a degreeof thiolation in the range of about 8004 to 250 μM per g polymerinteracted strongly with the mucins as evidenced by a broad linearviscoelastic region and the large difference between the storage modulusG′ and the loss modulus G″. One example is shown in FIG. 8 (content offree thiol groups 80 μmol/g).

The above described effect was dependent on the degree of thiolation.Unmodified chitosan HCl and chitosan-NAC with a degree of modificationof 4004 thiol groups/g polymer showed only weak interaction with themucins as evidenced by a non-existent and very narrow linearviscoelastic range, respectively. Results are shown in FIG. 6 (nothiolation, unmodified chitosan.HCl), FIG. 7 (content of free thiolgroups 40 μmol/g) and FIG. 8 and described in Table 6. Only when thedegree of thiolation was about 80 μM/g polymer the chitosan-NAC began toexhibit strong mucoadhesive properties as evidenced by stronginteraction with mucins.

TABLE 6 Determination of polymer-mucin interaction strength Content offree thiol groups Deformation γ (LVE range) Raw material [μmol/gthiomer] [%] Chitosan•HCl — — Chitosan-NAC 40 15 Chitosan-NAC 80 90

Example 9: Determination of Ocular Residence Time of Aqueous OphthalmicSolutions Containing 0.1% Radiolabelled Chitosan-NAC after Single OcularInstillation in Rabbits

Aim of the study was to obtain pharmacokinetic data of ¹²⁴I labeledchitosan-NAC at a concentration of 0.1% (w/w) after single topicalapplication of the test substance in a physiological buffer solutioninto the eye of female New Zealand white rabbits. Within this pilotstudy all of the four test animals received a single topicalinstillation of ¹²⁴I-Chitosan-NAC 0.1% (w/w) into the right eye. Ascontrols, two animals received additionally a single topical applicationof ¹²⁴I-Chitosan-HCl 0.1% (w/w) into the left eye and the remaining twoanimals received buffered ¹²⁴I-NaI into the left eye. Dynamic microPETmeasurements (350-650 keV energy window, 6 ns timing window) wereperformed for 1 hour after the administration of the test substances.Additionally, 15-min static scans were performed at 3, 6, and 9 hours,30-min static scans were performed at 24 hours and 60-min static scanswere performed 48, 72 and 96 hours after test substance administration.As main outcome parameter the radioactivity concentrations at theapplication sites were monitored. A calibration factor for convertingunits of microPET images into absolute radioactivity concentrations wasfirst generated by measuring a calibration cylinder phantom filled witha known radioactivity concentration of Na¹²⁴I solution. Radioactive testsubstance concentration was quantified from each image using the imageanalysis software AMIDE2. An ellipsoidal region of interest (ROIs) waspositioned at the edge of the eyes on the sagittal transmission image,whereby the area with high activity uptake located in the caruncle(inner corner of the eye) was excluded from the ROI. From the definedROIs time-radioactivity concentration curves (TACs), which means theradioactivity concentration (μCi/g) as a function of time in the tissuedefined by the ROI were calculated. The percent of applied dose per gramtissue (% AD/g) was calculated as average tissue concentration (μCi/g)divided by the totally applied radioactivity (μCi) at the start of theexperiment. Area-under the curve values were calculated from thetime-radioactivity concentration curves using PRISM5 software (GraphPadSoftware, La Jolla, Calif.).

As an additional quality control parameter thiol group content in the¹²⁴I labeled chitosan-NAC solution was measured before administration.

Results are shown in FIG. 9. Na¹²⁴I was rapidly cleared from the eye andhardly detectable at 6 hours after application. Single administration of0.1% (w/w)¹²⁴I-Chitosan-NAC solutions resulted in increased activityconcentrations at the administered eye as compared to ¹²⁴I-Chitosan-HClindicating an increase in mucoadhesive properties. At 24 hours afteradministration, activity concentration in the eyes treated with 0.1%(w/w) ¹²⁴I-Chitosan-NAC was almost 2.2-fold higher compared to the¹²⁴I-Chitosan-HCl eyes (2.16±0.36 vs 1.21±0.26% AD/g, respectively).This difference was maintained for the next observation period so thatat 48 hours after administration 2.35±0.15% AD/g (¹²⁴I-Chitosan-NAC) vs.1.06±0.30% AD/g (¹²⁴I-Chitosan-HCl) were found. Activity concentrationsfor both applied test substances did then decrease and were almostcomparable at 96 hours after administration.

Determination of the thiol group content for the ¹²⁴I labeledchitosan-NAC solutions used for this study (two solutions prepared ontwo different days) showed that about 110 μM and 130 μM thiol groups/gpolymer, respectively, remained in the free, i.e. unoxidized form afterradiolabelling with Bolton-Hunter reagent.

Example 10: Comparison of the Ocular Residence Time of AqueousOphthalmic Solutions Containing 0.1% Radiolabelled Chitosan-NAC or 0.1%Radiolabelled HA-Cysteamine after Single Ocular Instillation in Rabbits

In addition to the aforementioned study pharmacokinetic data wereobtained for ¹²⁴I-labelled Chitosan-NAC and ¹²⁴I-labelled thiolatedhyaluronan (HA-cysteamine) after topical application of the testsubstance at a concentration of 0.1% (w/w) in a physiological buffersolution into the eye of female New Zealand white rabbits. Within thisstudy test subjects received a single topical instillation of either0.1% (w/w) ¹²⁴I-HA-cysteamine (n=3) or 0.1% (w/w) ¹²⁴I-Chitosan-NAC(n=2) into the right eye while the left eye was kept untreated. RepeatedmicroPET measurements were performed for up to 3 days (72 hours) afterapplication of the test substances. As main study parameter theradioactivity concentrations at the application sites were monitored.Therefore, dynamic microPET measurements (350-650 keV energy window, 6ns timing window) were performed for 1 hour after the administration ofthe test substances. Additionally, 15-min static scans were performed at6 hours, 30-min static scans were performed at 24 hours, and 60-minstatic scans were performed 48 hours and 72 hours after test substanceadministration.

Quantitative analysis of obtained microPET pictures was performed asdescribed above and results shown in FIG. 10 are given as area-under thecurve (AUC) values calculated from the time-radioactivity concentrationcurves (% AD*h/g). Determination of the thiol group content in the ¹²⁴Ilabelled chitosan-NAC and ¹²⁴I-HA-cysteamine solutions used for thisstudy showed that about 154 μM and 54 μM thiol groups/g polymer,respectively, remained in the free, i.e. unoxidized form afterradiolabelling.

Example 11: Comparison of the Pharmacokinetic Behaviour of ¹²⁴I LabeledChitosan-NAC Solutions after Single Ocular Instillation in Rabbits

The ocular distribution of ¹²⁴I labeled chitosan-NAC solutionscontaining ¹²⁴I labeled chitosan-NAC in concentrations of 0.05%; 0.1%;0.3% and 0.5% (w/w), respectively, after a single instillation wasevaluated via qualitative assessment of microPET Scan projection imagesof summed data at different time points. The results are listed in Table7.

TABLE 7 Qualitative assessment of the ocular distribution of 4 differentradioactive labeled chitosan-NAC solutions Concentration (% w/w) of ¹²⁴Ilabeled chitosan-NAC in the ophthalmic test Qualitative QualitativeQualitative solution assessment t = 0-1 h assessment t = 6 h assessmentt = 22-24 h 0.05%  Even distribution on Even distribution on Notdetermined ocular surface, ocular surface; clear notable accumulationreduction of activity in tear duct as compared to t = 0-1 (clearance)0.1% Even distribution on Even distribution on Even distribution ocularsurface, some ocular surface on ocular surface accumulation in tear duct(clearance) 0.3% Even distribution on Even distribution on Evendistribution ocular surface ocular surface on ocular surface 0.5% unevendistribution on Uneven distribution Uneven ocular surface with on ocularsurface with distribution on highest radioactivity highest radioactivityocular surface concentrations in concentrations in with highestconjunctival sac and conjunctival sac and radioactivity inner canthus ofthe inner canthus of the concentrations in eye eye; clearly reducedconjunctival sac activity compared to and inner canthus 0.3% testsolution at of the eye; the same time point reduced activity compared to0.3% test solution at the same time point

The qualitative assessment of ocular surface distribution of 4 differentradioactive labelled chitosan-NAC solutions clearly showed thatsolutions with concentrations of 0.1% and 0.3% chitosan-NAC showed thebest results regarding even distribution on the ocular surface ofrabbits for 24 h. The solution containing 0.05% was cleared more rapidlyfrom the ocular surface. The solution containing 0.5% chitosan-NAC wasnot as evenly distributed on the eye and also was cleared more rapidlyfrom the ocular surface.

Example 12: Effect of Chitosan-NAC on Tear Film Thickness in Patientswith Dry Eye Syndrome

The aim of this study was to investigate the effect of eye dropscontaining 0.1% chitosan-NAC (171 μmol/g free thiol groups) on tear filmthickness in patients with dry eye syndrome after a single dose andafter 5 day treatment. For this purpose 2 cohorts were planned: InCohort I, chitosan-N-acetylcysteine eye drops were instilled once in onerandomly chosen eye, whereas the fellow eye received placebo.Measurements of tear film thickness were performed with opticalcoherence tomography (OCT) before instillation and 10 minutes, 1 hour, 2hours, 4 hours, 8 hours, 10 hours, 12 hours and 24 hours afterinstillation. The results (median tear film thickness in μm, range inparentheses, n=16) are shown in Table 8.

TABLE 8 Results of tear film thickness measurements of cohort I TimeChitosan-NAC Placebo pre-dose 3.9 (3.3-4.8) 4.0 (3.0-5.3) 10 min 4.7(3.4-7.0) 4.2 (2.0-6.3)  8 h 4.7 (3.5-6.5) 4.2 (3.0-6.5) 10 h 5.2(3.5-7.9) 4.3 (3.3-5.5) 12 h 4.6 (3.7-7.6) 4.1 (2.9-6.9) 24 h 4.7(4.0-8.1) 4.7 (2.8-6.3)

In Cohort II, 16 Patients were randomized to receive chitosan-NAC eyedrops 0.1% once a day in the right eye and twice a day in the left eyeor vice versa. The other eye received placebo for the secondinstillation. Measurements of tear film thickness with OCT wereperformed every study day before the morning instillation and the dayafter the last instillation. The Ocular Surface Disease Index (OSDI) wasassessed as secondary efficacy variable in cohort II at screening and atday 6.

The results of tear film thickness measurements in cohort II are shownin Table 9 (median tear film thickness (μm) of “once daily” and “twicedaily” group in Cohort II; range in parentheses, n=16).

TABLE 9 Results of tear film thickness measurements of cohort II StudyDay twice daily (BID) once daily (QD) 1 3.7 (2.7-6.1) 3.4 (2.8-6.0) 24.3 (2.7-6.6) 3.9 (2.7-7.5) 3 4.1 (3.0-7.4) 3.9 (2.9-8.5) 4 4.3(2.7-6.9) 4.1 (2.5-7.2) 5 4.4 (2.9-7.7) 4.1 (3.3-8.4) 6 4.4 (3.3-7.3)4.0 (2.9-7.2)

Based on a regression analysis over the entire 24 hr post-dose period,the increase in tear film thickness was statistically significant if thetreatment group of cohort I was compared with the placebo group(Regression analysis to identify between-treatment differences overtime, p<0.0001). These results show that chitosan-NAC eye drops caused astatistically significant increase of tear film thickness over a timeperiod of 24 h, suggesting a long corneal residence time. Tear filmthickness was restored to a level found in healthy subjects(Werkmeister, Alex et al., 2013, Measurement of tear film thicknessusing ultrahigh-resolution optical coherence tomography, InvestOphthalmol Vis Sci (54): 5578-5583). In cohort II once dailyinstillations were sufficient to restore tear film thickness and werenot inferior to twice daily applications.

In Cohort II the median ocular surface disease index at screening was38.5 and improved to 13.0 at day 6, which corresponds to a decrease by60.5%. This indicates that the subjective severity of DES as assessedwith the OSDI improved after five days of treatment with chitosan-NACeye drops 0.1%.

The invention claimed is:
 1. A method of treating dry eye syndrome, adry eye sign, or a dry eye symptom, comprising administering a sterileaqueous ophthalmic solution to an eye for the treatment of dry eyesyndrome, a dry eye sign, or a dry eye symptom, wherein the sterileaqueous ophthalmic solution comprises: a carrier solution; and about0.05 to about 0.5% (w/w) of N—(N-acetylcysteinyl-)chitosan or apharmaceutically acceptable salt thereof, wherein theN—(N-acetylcysteinyl-)chitosan or pharmaceutically acceptable saltthereof has a content of free thiol groups in an amount of from 80μmol/g polymer to 280 μmol/g polymer.
 2. The method of claim 1, whereinsaid dry eye signs or symptoms is caused by or associated with at leastone of: keratoconjunctivitis sicca (KCS), age-related dry eye,Stevens-Johnson syndrome, Sjogren's syndrome, ocular cicatricalpemphigoid, corneal injury, infection, Riley-Day syndrome, congenitalalacrima, photorefractive keratectomy (PRK) laser eye surgery,laser-assisted sub-epithelial keratectomy (LASEK), laser-assisted insitu keratomileusis (LASIK), allergic conjunctivitis, blepharitis andMeibomian gland dysfunction, nutritional disorder or deficiency, vitamindeficiency, pharmacologic side effect, glandular and tissue destruction,autoimmune or immunodeficient disorder, contact lens intolerance,environmental exposure to airborne particulates, environmental exposureto smoke, environmental exposure to smog, environmental exposure toexcessively dry air, inability to blink in a comatose patient, or eyestress caused by computer work or computer gaming.
 3. The method ofclaim 1, wherein the sterile aqueous ophthalmic solution is administeredto the eye no more than twice a day.
 4. The method of claim 1, whereinthe sterile aqueous ophthalmic solution is administered to the eye nomore than once a day.
 5. The method of claim 1, wherein thepharmaceutically acceptable salt is selected from the group consistingof salts of organic acids and salts of mineral acids.
 6. The method ofclaim 1, wherein the N—(N-acetylcysteinyl-)chitosan or pharmaceuticallyacceptable salt thereof has a content of crosslinked thiol groups thatis 30% or less of the total thiol groups therein, the total thiol groupstherein including crosslinked and free thiol groups.
 7. The method ofclaim 1, further comprising at least one excipient selected from thegroup consisting of boric acid, salts of boric acid, salts of citricacid, salts of acetic acids, polyethylene glycol, Na₂-EDTA, mannitol,sorbitol, glycerol, sodium chloride, sodium metabisulfite, hydroxypropylmethylcellulose, polyvinylpyrrolidone, polyvinylalcohol, lubricin,hyaluronic acid, pharmaceutically acceptable salts of hyaluronic acid,and mixtures thereof.
 8. The method of claim 1, wherein theN—(N-acetylcysteinyl-)chitosan or pharmaceutically acceptable saltthereof used for the preparation of the solution exhibits at aconcentration of 0.5% in water at 25° C. a kinematic viscosity withinthe range of 1 to 15 mm²/s, wherein the kinematic viscosity is measuredusing a concentration of 0.5% N—(N-acetylcysteinyl-)chitosan orpharmaceutically acceptable salt thereof in water.
 9. The method ofclaim 1, wherein the solution exhibits an osmotic pressure of from 150to 400 mOsM.
 10. The method of claim 1, wherein the solution exhibits apH-value of from 5.8 to 6.8.
 11. The method of claim 1, wherein theN—(N-acetylcysteinyl-)chitosan or pharmaceutically acceptable saltthereof has a content of crosslinked thiol groups that is 30% or less ofthe total thiol groups therein, the total thiol groups therein includingcrosslinked and free thiol groups, after being stored for at least 12months at room temperature.
 12. A method of treating dry eye syndrome, adry eye sign, or a dry eye symptom, comprising administering a sterileaqueous ophthalmic solution to an eye for the treatment of dry eyesyndrome, a dry eye sign, or a dry eye symptom, wherein the sterileaqueous ophthalmic solution comprises: a carrier solution; and about0.05 to about 0.5% (w/w) of N—(N-acetylcysteinyl-)chitosan or apharmaceutically acceptable salt thereof, wherein theN—(N-acetylcysteinyl-)chitosan or pharmaceutically acceptable saltthereof has a content of free thiol groups in an amount of from 80μmol/g polymer to 280 μmol/g polymer, and wherein theN—(N-acetylcysteinyl-)chitosan or pharmaceutically acceptable saltthereof has a content of crosslinked thiol groups that is 30% or less ofthe total thiol groups therein, the total thiol groups therein includingcrosslinked and free thiol groups, after being stored for at least 12months at room temperature.
 13. A method of treating dry eye syndrome, adry eye sign, or a dry eye symptom, comprising administering a sterileaqueous ophthalmic solution to an eye for the treatment of dry eyesyndrome, a dry eye sign, or a dry eye symptom, wherein the sterileaqueous ophthalmic solution comprises: a carrier solution; and about0.05 to about 0.5% (w/w) of a pharmaceutically acceptable salt ofN—(N-acetylcysteinyl-)chitosan, the N—(N-acetylcysteinyl-)chitosan beingselected from the group consisting of a salt of an organic acid or asalt of a mineral acid, wherein the N—(N-acetylcysteinyl-)chitosan has acontent of free thiol groups in an amount of from 80 μmol/g polymer to280 μmol/g polymer.
 14. The method of claim 5, wherein the salts oforganic acids comprise one or more of acetic acid, citric acid, formicacid, or tartaric acid.
 15. The method of claim 5, wherein the salts ofmineral acids comprise one or both of HCl or H₂SO₄.